The present invention relates to an anti-ringing device for a television signal transmission system and, particularly, to such a device capable of changing the band limiting characteristics thereof with respect to a luminance component of a video signal to prevent ringing from occuring during reception of the video signal.
In a television transmission signal in the NTSC system, a band limitation is performed for a luminance signal. That is, when a video signal obtained by a high resolution camera is received, ringing may be produced in various areas on a screen. Therefore, it is desired to limit the band preliminarily in a transmission system such that the production of such ringing is prevented and to prevent the resolution from being degraded. The term "ringing" used in this description means a local change of an image on a screen caused by an abrupt change of luminance due to the high frequency cutoff characteristics of a transmission line. That is, the ringing on the image screen takes the form of thin white or black contour lines surrounding a portion of a picture on the screen. Therefore, such ringing may or may not be conspicuous depending upon the relation of the picture portion to the background thereof. For example, when the picture portion is a thin tower with a blue sky as the background, the ringing can not be ignored. However, when the background thereof is woods, it may be ignored.
In the NTSC television signal transmission system, the frequency band of the luminance signal is limited to 4.2 MHz, and the color signal frequency bands for the I signal and the Q signal are limited to 1.5 MHz and 0.5 MHz, respectively. Further, a reference sub-carrier frequency of 3.58 MHz is mixed thereto and a resultant signal is fed to a transmitter. Thus, a video signal obtained from an image device is band-limited by passing it through a low-pass filter and then fed to the transmission system which may include a signal recording device such as a VTR.
Conventional low-pass filters used to limit the luminance signal frequency band have such characteristics as shown in FIGS. 3A and 3B, in which FIG. 3A is a usual low pass filter designed to improve image quality and FIG. 3B is a filter designed to remove ringing. In FIG. 3A, the characteristic is substantially flat up to a frequency of 4.2 MHz which is very close to an upper frequency limitation of the frequency band, and has a sharp cutoff above 4.2 MHz. Therefore, the amplitude limitation of the luminance signal in a high frequency region of the video signal from the image device is small, and thus the image on the screen becomes sharp. When a high resolution camera is used as the image device on the transmission side, signal component up to 6 to 7 MHz can be obtained, and ringing is produced at various points on the screen.
On the other hand, in FIG. 3B, the characteristic slopes down gradually at frequencies higher than, for example, 3 MHz. It has been found that, with such a filter, the production of ringing is minimized. However, since the amplitude of the 4 MHz signal is reduced, blurring of the whole image occurs due to degradation of resolution.
The sharpness of the image on a screen is thus contradictory to the conspicuousness of ringing, and it is very difficult to make both of them optimum.